The present invention relates to a code division multiple access (CDMA) transmission technique, and more particularly to, a modulation technique for a case in which a Walsh sequence having a relatively long code length is used, such as an m-ary modulation, in a radio receiver.
The CDMA transmission technique is one of multiple access communication techniques used for sharing a limited frequency band by a plurality of users, and is known for a high frequency-utilization efficiency compared with time division multiple access (TDMA) and frequency division multiple access (FDMA) transmission techniques. In CDMA, data obtained after modulation is multiplied by a pseudo-random code called a PN code and is transmitted. The multiplication process by the PN code is called spreading, and has an important role of identifying the user in CDMA. The PN code has a sufficiently high transmission rate (approximately 100 to 1000 times) compared with the bit rate of generally transmitted information. Therefore, by performing the spreading process, the bandwidth of a transmission signal is extended to 100 to 1000 times that obtained before the spreading process. In the PN code, since a different code sequence is used for each user, a multiplication process by the same PN sequence as in the transmission side is performed in a demodulation process. This is called a despreading process. The despreading process by the same code sequence as in the transmission side is performed to regenerate the original information bit. In this case, whether the information bit is “1” or “0” is determined by calculating a correlation with a spreading code. This is because calculating the correlation reduces the effect of user signals having different code sequences, and allows only a signal having the same code sequence to be extracted.
The ratio of the spreading-code rate to the information-bit rate is called the processing gain. In CDMA, the larger the processing gain is, the better the communication. Therefore, to improve communication quality in CDMA, it is necessary to increase the spreading-code rate or to reduce the transmission-data rate. In the m-ary modulation technique, the processing gain can be increased without reducing the transmission-data rate. With this characteristic, m-ary modulation is frequently used in CDMA communications.
In EIA/TIA/IS-95A (hereinafter called IS-95A) known as “cdmaOne,” 64-ary modulation using a code length of 64 is used in a reverse link (transmission at a mobile station and receiving at a base station). The 64-ary demodulation method used in IS-95A is described in detail in U.S. Pat. No. 5,442,627, “Noncoherent Receiver Employing a Dual Maxima Metric Generation Process” by Audrey Viterbi and Andrew J. Viterbi. Since a pilot channel is not transmitted in the reverse link of IS-95A, it is difficult to perform synchronous detection for estimating the phase of a signal during demodulation. Therefore, U.S. Pat. No. 5,442,627 shows a structure used in a case in which demodulation is performed by asynchronous detection, where phase estimation is not performed. In this case, to demodulate a signal to which 64-ary modulation has been applied, an Hadamard transform is first applied to an I phase and a Q phase, and then the correlation between the received signal and 64 Walsh code sequences is calculated. The data obtained by correlation is converted to electric power by calculating the sums of squares of the I-phase component and the Q-phase component. The electric power is compared with that of each Walsh code to determine the Walsh code having the maximum value to determined the transmitted Walsh code.